Hilo

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms early in the evening, then numerous showers in the late evening and overnight. Lows 62 to 69 near the shore to 51 to 57 at 4000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Monday: Partly sunny with scattered showers. Highs 79 to 85 near the shore to 64 to 69 at 4000 feet. North winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with numerous showers. Lows 62 to 70 near the shore to 53 to 58 at 4000 feet. North winds up to 10 mph. Chance of rain 70 percent.

Kona

Tonight: Mostly cloudy early in the evening then becoming partly cloudy. Numerous showers and slight chance of thunderstorms early in the evening, then isolated showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows around 73 near the shore to 47 to 53 near 5000 feet. Light winds. Chance of rain 70 percent.

Monday: Mostly sunny. Isolated showers in the afternoon. Highs 84 to 89 near the shore to around 68 near 5000 feet. West winds up to 10 mph. Chance of rain 20 percent.

Monday Night: Partly cloudy. Isolated showers in the evening. Lows around 73 near the shore to 48 to 54 near 5000 feet. Light winds. Chance of rain 20 percent.

Waimea

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms early in the evening, then scattered showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows 63 to 71 near the shore to 54 to 61 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Monday: Breezy. Partly sunny with scattered showers. Highs 65 to 85. East winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 62 to 71 near the shore to 54 to 61 near 3000 feet. East winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain 50 percent.

Kohala

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms early in the evening, then scattered showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows 63 to 71 near the shore to 54 to 61 near 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Monday: Breezy. Partly sunny with scattered showers. Highs 65 to 85. East winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 62 to 71 near the shore to 54 to 61 near 3000 feet. East winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain 50 percent.

South Big Island

Tonight: Mostly cloudy early in the evening then becoming mostly clear. Breezy. Numerous showers and slight chance of thunderstorms early in the evening. Lows around 74 near the shore to around 51 near 5000 feet. Northeast winds up to 20 mph. Chance of rain 70 percent.

Monday: Sunny and breezy. Isolated showers in the afternoon. Highs around 86 near the shore to around 68 near 5000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Monday Night: Partly cloudy. Lows around 73 near the shore to around 52 near 5000 feet. Northeast winds 10 to 15 mph decreasing to up to 10 mph after midnight.

Puna

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms early in the evening, then numerous showers in the late evening and overnight. Lows 62 to 69 near the shore to 51 to 57 at 4000 feet. West winds up to 10 mph. Chance of rain 70 percent.

Monday: Partly sunny with scattered showers. Highs 79 to 85 near the shore to 64 to 69 at 4000 feet. North winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with numerous showers. Lows 62 to 70 near the shore to 53 to 58 at 4000 feet. North winds up to 10 mph. Chance of rain 70 percent.

Waikoloa

Tonight: Partly cloudy. Scattered showers and slight chance of thunderstorms early in the evening. Locally heavy rainfall possible early in the evening. Lows around 73 near the shore to 50 to 55 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Monday: Sunny and breezy. Highs 84 to 90 near the shore to 66 to 73 above 4000 feet. Northeast winds up to 20 mph.

Monday Night: Mostly clear. Lows 71 to 76 near the shore to 51 to 56 above 4000 feet. Light winds.

Detailed Forecast

Synopsis

A low aloft will support unstable conditions, a few heavier showers, and a slight chance of thunderstorms over the western end of the state though tonight. However, this upper low will weaken and shift east of the islands on Monday as high pressure at the surface builds back into the region from the northwest. This will allow moderate to locally breezy trade winds to gradually build across the islands, marking the return of a more typical trade wind pattern. Showers and clouds will generally favor windward and mountain locations for the remainder of the week.

Discussion

Afternoon water vapor imagery shows that the upper low that has been moving southward over the western end of the state is now located just south of Oahu as of mid afternoon. The 00z sounding from Lihue continues to show a relatively unstable atmosphere, with convective available potential energy (CAPE) increasing slightly since last night as the upper low has moved into closer proximity. The sounding also shows that the temperature inversion has largely eroded, which has resulted in some towering cumulus development and heavier showers over the coastal waters south of Kauai this afternoon. GOES-W Geostationary Lightning Mapper (GLM) has also detected several lightning flashes in this area where instability and lift are maximized. With that said, fairly dry air in the mid levels and near-normal precipitable water (1.2 inches) have continued to limit more widespread heavy shower development and thunder potential. Regardless, latest observations, data, and guidance all warrant continuing to mention a slight chance of thunderstorms and the possibility for brief heavy showers through tonight.
Meanwhile, weakened ridging north of the islands has resulted in gentle to moderate trade winds prevailing across the state today. This has allowed sea breezes to develop in select leeward locations, resulting in afternoon cloud buildups and a few showers along leeward and interior sections of the islands. These are expected to clear tonight. Trades may still be light enough to support overnight land breezes, but are generally expected to start building back over the state tonight through early Monday. Therefore, expect chances for heavier showers to shift back towards windward and mountain locations, but moisture availability will continue to be a limiting factor.
Guidance continues to suggest that trade winds gradually increase on Monday as a new high pressure to the distant northwest builds eastward. Instability may linger through Monday, but generally, increasingly stable conditions should return by Tuesday at the latest and persist through much of the week as the upper low weakens into a broad trough and shifts just east of the state. With the return of a more typical trade wind pattern, expect clouds and showers to favor primarily windward and mauka areas. With this afternoon's forecast updates, shower chances were tweaked for the remainder of the week based on the latest guidance, but the overall forecast philosophy remains unchanged.

Aviation

Moderate trade winds will continue through tonight before increasing to moderate to locally breezy Monday. A weak upper level low moving over the islands will increase low cloud and showers. Periods of MVFR conditions will favor windward and mountain areas, including converging sea breeze boundaries along the leeward slopes. There remains a slight chance of thunderstorms over the state tonight.
No AIRMETs are currently in effect.

Marine

An upper low currently centered near Oahu will keep a chance for thunderstorms in the forecast for the offshore and coastal waters near the low through tonight, before it lifts back to the north. A surface ridge will rebuild to the north of the islands tonight, strengthening trades across the region. By Monday morning, Small Craft Advisory (SCA) level winds are possible in the typical windier coastal waters near Maui County and the Big Island. Will monitor conditions and the latest high-res models overnight to determine the timing for issuing the SCA. SCA winds are expected to continue through at least mid-week.
A moderate, long-period north-northwest swell appears to be running a few hours later than forecast. However, some energy is finally beginning to show up in the 18-22 second period band at the Hanalei nearshore buoy this afternoon. Now expect this swell to build tonight and peak Monday, then slowly decline through the week. Surf is projected to stay a few feet below advisory levels for north facing shores with this swell. A medium to long- period south swell is filling in this afternoon as well, and should hold around 3 feet/14 seconds through Monday. This swell may boost surf along south facing shores to near or slightly below advisory levels during its peak tonight. This swell will decline over the western half of the island chain through the middle of the week, while holding fairly steady over the eastern half. Small, background south swell energy is expected for the remainder of the week. East facing shores will get a boost in choppy surf as the trades return this week.

HFO Watches/Warnings/Advisories

None.

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Data Courtesy of NOAA.gov